A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Radiation that has been patterned by the mask is focussed onto the substrate using a projection system. The optical properties of the projection system may depend on the polarization state of the radiation which is projected by the projection system. It is desirable to determine the dependence of one or more optical properties of the projection system on the polarization state of the radiation which is projected by the projection system.
It is an object of the present invention to provide an apparatus and a method which at least partially address one or more of the problems of the prior art, whether identified herein or elsewhere.